I. Field of the Invention
This invention pertains to doped fiber amplifiers for inline use in fiber optic communication systems. More particularly, the present invention pertains to a two-stage in-line erbium doped fiber amplifier (EDFA) system for adding and dropping telemetry signals from a fiber optic line. Most particularly, the present invention pertains to a two-stage in-line EDFA system containing a filter for removing an existing telemetry signal from a fiber optic communication line and a means for adding a new telemetry signal to a fiber optic communication line without affecting the data carrying capacity of the line. In addition, this invention pertains to a method of monitoring the performance of fiber optic communication systems and for locating faults within such systems.
II. Background Art
Commercially available regenerated fiber optic communication systems use a plurality of regenerators, which include receiver/transmitter pairs and accompanying electronic processing circuitry. The regenerators are positioned between links in an optical fiber line which, when combined, form a chain, and regenerate the signal carried on the optical fiber because the signal suffers attenuation as it travels down the chain. The receiver stage of a regenerator receives the optical data carried on the optical fiber whereupon the data is demultiplexed and converted to digital electronic signals for processing. The processing usually includes filtering out the noise in the signals and re-timing the signals. Once the signals are processed, they are multiplexed, amplified, converted back to optical data, and then re-transmitted by the transmitter stage of the regenerator. The newly transmitted data will travel along the next link until it reaches the next regenerator. In such systems, the electronic processing circuitry is also used to monitor the performance of the system by monitoring the converted optical signal as well as the receiver and transmitter stages of the regenerators.
As optical signals travel between links in a fiber optic system, the signals suffer attenuation, partly as a result of inherent optical fiber characteristics such as bending losses, connection losses, etc. Thus, in optical regenerated systems, the distance between adjacent regenerators is limited so that the optical signals will be of sufficient strength to be processed upon reaching the next regenerator, whereupon the optical signals are again demultiplexed, converted to electric signals and cleaned up by removing noise. The electric signals are then amplified, retimed, converted back to optical signals and re-transmitted. This results in the need for many regenerators, thereby increasing the cost of such systems.
The use of erbium doped fiber amplifiers (EDFAs) as replacements for regenerators in fiber optic systems is well known to those of ordinary skill in the art. EDFAs optically amplify the signal carried on the optical fiber. Such optically amplified systems are analog and not digital. Therefore, the information bits that are used for telemetry and performance monitoring in the digital regenerated systems described above are not available. In addition, EDFAs generate amplifier spontaneous emission (ASE) noise which can cause degradations in the system, especially as the ASE noise is further amplified by subsequent EDFAs in the chain. Thus, it would be beneficial in fiber optic communication systems using EDFAs to have a telemetry signal transmitted along with the data signal for transporting telemetry data and monitoring the performance and various characteristics of a system without the telemetry signal adversely impacting the data carrying capacity of the system.
Accordingly, it is an object of the present invention to provide a two-stage EDFA system for use in regenerated fiber optic communication systems for adding and dropping a telemetry signal without affecting the data carrying capacity of the communication system.
It is a further object of the present invention to provide a two-stage EDFA system comprising a filter for removing the forward propagating and backward propagating amplifier spontaneous emission noise generated by the EDFAs, thereby increasing the signal to noise ratio of the communication system.
It is still a further object of the present invention to provide a method for adding and dropping a telemetry signal from an optical fiber communication system as well as a method for monitoring system performance and fault location.
Other objects will become apparent as the following description proceeds.
The foregoing as well as additional details of the present invention will be more fully apparent from the following detailed description and annexed drawings of the presently preferred embodiment thereof.